linux-kernel - Re: [PATCH v2 8/9] PCI: dwc: Small computation improvement

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Date:   Mon, 16 Apr 2018 09:41:11 +0100
From:   Gustavo Pimentel <gustavo.pimentel@...opsys.com>
To:     Jingoo Han <jingoohan1@...il.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "Joao.Pinto@...opsys.com" <Joao.Pinto@...opsys.com>,
        "kishon@...com" <kishon@...com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>
Cc:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 8/9] PCI: dwc: Small computation improvement

Hi Jingoo,

On 11/04/2018 20:37, Jingoo Han wrote:
> On Wednesday, April 11, 2018 3:40 AM, Gustavo Pimentel wrote:
>>
>> Hi Jingoo,
>>
>> On 11/04/2018 01:01, Jingoo Han wrote:
>>> On Monday, April 9, 2018 5:41 AM, Gustavo Pimentel wrote:
>>>>
>>>> Replaces a simple division by 2 to a right shift rotation of 1 bit.
>>>
>>> It looks good. However, would you add a simple reason to the commit
>>> message?
>>
>> Sure.
>>
>> Can be this one?
>>
>> Probably any recent and decent compiler does this kind of substitution
>> in order to improve code performance. Nevertheless it's a coding good
>> practice whenever there is a division / multiplication by multiple of 2
>> to replace it by the equivalent operation in this case, the shift
>> rotation.
> 
> Yes, that's what I wanted. The most platforms using 'dwc' are based on
> ARM CPUs. So, the shift rotation can be better.
> Thank you.

Can I get your acked-by on this?

> 
> Best regards,
> Jingoo Han
> 
>>
>>>
>>> Best regards,
>>> Jingoo Han
>>>
>>>>
>>>> Signed-off-by: Gustavo Pimentel <gustavo.pimentel@...opsys.com>
>>>> ---
>>>> Change v1->v2:
>>>> - Nothing changed, just to follow the patch set version.
>>>>
>>>>  drivers/pci/dwc/pcie-designware-host.c | 8 ++++----
>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/pci/dwc/pcie-designware-host.c
>>>> b/drivers/pci/dwc/pcie-designware-host.c
>>>> index 03e9b82..8e6fed4 100644
>>>> --- a/drivers/pci/dwc/pcie-designware-host.c
>>>> +++ b/drivers/pci/dwc/pcie-designware-host.c
>>>> @@ -332,8 +332,8 @@ int dw_pcie_host_init(struct pcie_port *pp)
>>>>
>>>>  	cfg_res = platform_get_resource_byname(pdev, IORESOURCE_MEM,
>>>> "config");
>>>>  	if (cfg_res) {
>>>> -		pp->cfg0_size = resource_size(cfg_res) / 2;
>>>> -		pp->cfg1_size = resource_size(cfg_res) / 2;
>>>> +		pp->cfg0_size = resource_size(cfg_res) >> 1;
>>>> +		pp->cfg1_size = resource_size(cfg_res) >> 1;
>>>>  		pp->cfg0_base = cfg_res->start;
>>>>  		pp->cfg1_base = cfg_res->start + pp->cfg0_size;
>>>>  	} else if (!pp->va_cfg0_base) {
>>>> @@ -377,8 +377,8 @@ int dw_pcie_host_init(struct pcie_port *pp)
>>>>  			break;
>>>>  		case 0:
>>>>  			pp->cfg = win->res;
>>>> -			pp->cfg0_size = resource_size(pp->cfg) / 2;
>>>> -			pp->cfg1_size = resource_size(pp->cfg) / 2;
>>>> +			pp->cfg0_size = resource_size(pp->cfg) >> 1;
>>>> +			pp->cfg1_size = resource_size(pp->cfg) >> 1;
>>>>  			pp->cfg0_base = pp->cfg->start;
>>>>  			pp->cfg1_base = pp->cfg->start + pp->cfg0_size;
>>>>  			break;
>>>> --
>>>> 2.7.4
>>>>
>>>
>>>
>>
>> Regards,
>> Gustavo
> 
> 

Regards,
Gustavo